In communication using IPv6 (Internet Protocol version 6), Neighbor Cache is used for translation of an IP address (layer 3) and a link layer address (layer 2). A management control method for Neighbor Discovery and Neighbor Cache entry state in Neighbor Cache is described in Non Patent Literature 1 (RFC4861). In existing communication systems using IPv6, a Neighbor Discovery message is used as a message for resolving and managing the correspondence between IP addresses and link layer addresses. Further, Neighbor Cache is used as a table storing the correspondence between IP addresses and link layer addresses. Although the present invention is intended for general communication systems using IPv6, it is a technique that is significantly effective particularly for a communication system in which address update occurs frequently as described in Non Patent Literature 2.
In IPv6, update of Neighbor Cache is typically performed in compliance with RFC4861. However, RFC4861 is lacking in a structure to delete an IP address from Neighbor Cache when the IP address is no longer required. This is because, as the way to use an IP address, permanent use has been common, and it has been rare to encounter the occasion where an IP address becomes no longer required, and even when such a case occurs, it is not recognized, and therefore this problem has not become apparent. However, with the recent proliferation of the mobile communication environment, change of connections is made frequently, occasions to make a terminal enter suspend or sleep mode increase, which causes the state where Neighbor Cache for the terminal no longer used is accumulated to occur. Neighbor Cache not in use leads to network vulnerability and is thus a problem.
In light of this, an example of a management control method of Neighbor Cache that deletes Neighbor Cache no longer used is described in Patent Literature 1. A connection management method described in Patent Literature 1 is the connection management method of a router device that performs connection management of a communication terminal connected to a link in an IP network, which includes a step in which the communication terminal notifies the router device about its own communication status, a step in which when the communication status is notification of a communication disabled state, the router device that has received the notification causes nearby cache (Neighbor Cache) entries around the communication terminal to transition to the communication disabled state, and a step in which when the router device receives a packet addressed to the communication terminal, the router device checks the state of nearby cache entries and, if it is the communication disabled state, immediately transmits the transfer error message to a sender of the packet. The connection management method described in Patent Literature 1 further includes a step in which the communication terminal clears the link connection after giving a notification of the communication disabled state to a first router device and, when the communication terminal connects to a link managed by a second router device, the communication terminal requests the first router device to delete the nearby cache entry. Then, the router device deletes the entry of the communication terminal in the nearby cache after the lapse of a specified period from the receipt of the notification of the communication disabled state from the communication terminal.